1. Field of the Invention
The present invention is directed to an on-board gas composition sensor for monitoring oxygen content levels in the exhaust gas from an internal combustion engine. More particularly, the invention is related to an optic sensor element responsive to changing oxygen concentration and a process of making this element using sol-gel techniques whereby a fluorescent indicator is included in a metal oxide matrix.
2. Background of the Invention
Motor vehicle manufacturers use on-board gas sensors to measure exhaust gases for engine control purposes and for assessing the performance of a catalytic converter. Sensors of this type must be sufficiently robust to withstand the aggressive gases to which the sensor is exposed in an exhaust gas conduit and also the extreme conditions that exist there such as high temperatures (typically of about 500.degree.-600.degree. C.), temperature cycling, vibration, intermittent moisture, etc.
Exhaust gas sensors used to date are disadvantaged by the need for an electrical signal which varies in relation to the oxygen content level of the exhaust gas. The sensor output signal may be used as an input to an electronic engine control means, such as for an electronic engine control module controlling the air/fuel mixture which is fed at any given point in time to a combustion chamber of the engine. Maintaining the integrity of the electrical connections to the sensor presents a challenge in the field of motor vehicle engine control. In addition, a problem is presented in maintaining the correct functioning of the active sensor material exposed to the harsh environment of the exhaust conduit.
Recently issued U.S. Pat. No. 5,490,490 discloses a different type of oxygen sensor suitable for operation at exhaust gas temperatures of 400.degree.-650.degree. C. which is to overcome the disadvantages of such prior sensors. It employs a bead of porous inorganic oxide such as Cu-ZSM-5 zeolite fused to the end of a fiber-optic cable. The bead is exposed to exhaust gas and the cable is connected to a device consisting of a light source and fluorescence detector. Upon irradiation with ultraviolet or blue light (350-460 nm), the bead exhibits fluorescence whose intensity is related to either oxygen concentration or reductant to oxidant ratio of the exhaust gases. Attaching a bead to the end of the fiber, however, involves manufacturing complexity and using a bead provides an opportunity for it to be dislodged from the fiber during use. In addition, such a bead has high temperature durability issues, since a thermal expansion mismatch with the silica fiber might cause it to crack and fall off. It would be desirable to provide an optical fiber/fluorescent indicator element which has improved high temperature resistance, durability, and response time, which can be manufactured in a commercially desirable way.
The present invention provides such an element. These and other advantages of the invention will be understood from the following disclosure and detailed description of certain preferred embodiments.